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Polypropylene

Chemical and physical properties

Micrograph of polypropylene

Most commercial polypropylene is isotactic and has an intermediate level of crystallinity between that of low density polyethylene (LDPE) and high density polyethylene (HDPE); its Young’s modulus is also intermediate. PP is normally tough and flexible, especially when copolymerized with ethylene. This allows polypropylene to be used as an engineering plastic, competing with materials such as ABS. Polypropylene is reasonably economical, and can be made translucent when uncolored but is not as readily made transparent as polystyrene, acrylic or certain other plastics. It is often opaque or colored using pigments. Polypropylene has good resistance to fatigue.

Polypropylene has a melting point of ~160 C (320 F), as determined by differential scanning calorimetry (DSC).

The MFR (melt flow rate) or MFI (melt flow index) is a measure of PP’s molecular weight. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease.

There are three general types of PP: homopolymer, random copolymer and block copolymer. The comonomer used is typically ethylene. Ethylene-propylene rubber or EPDM added to PP homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to PP homopolymer decreases the polymer crystallinity and makes the polymer more transparent.

Degradation

Polypropylene is liable to chain degradation from exposure to UV radiation such as that present in sunlight. Oxidation usually occurs at the secondary carbon atom present in every repeat unit. A free radical is formed here, and then reacts further with oxygen, followed by chain scission to yield aldehydes and carboxylic acids. In external applications, it shows up as a network of fine cracks and crazes which become deeper and more severe with time of exposure.

For external applications, UV-absorbing additives must be used. Carbon black also provides some protection from UV attack. The polymer can also be oxidized at high temperatures, a common problem during molding operations. Anti-oxidants are normally added to prevent polymer degradation.

History

Polypropylene was first polymerized by Karl Rehn and Giulio Natta in March 1954, serving as a preliminary work for large-scale synthesis from 1957 onwards. At first it was thought that it would be cheaper than polyethylene.

Synthesis

Short segments of polypropylene, showing examples of isotactic (above) and syndiotactic (below) tacticity.

An important concept in understanding the link between the structure of polypropylene and its properties is tacticity. The relative orientation of each methyl group (CH3 in the figure at left) relative to the methyl groups on neighboring monomers has a strong effect on the finished polymer’s ability to form crystals, because each methyl group takes up space and constrains backbone bending.

Like most other vinyl polymers, useful polypropylene cannot be made by radical polymerization due to the higher reactivity of the allylic hydrogen (leading to dimerization) during polymerization. Moreover, the material that would result from such a process would have methyl groups arranged randomly, so called atactic PP. The lack of long-range order prevents any crystallinity in such a material, giving an amorphous material with very little strength and only specialized qualities suitable for niche end uses.

A Ziegler-Natta catalyst is able to limit incoming monomers to a specific orientation, only adding them to the polymer chain if they face the right direction. Most commercially available polypropylene is made with such Ziegler-Natta catalysts, which produce mostly isotactic polypropylene (the upper chain in the figure above). With the methyl group consistently on one side, such molecules tend to coil into a helical shape; these helices then line up next to one another to form the crystals that give commercial polypropylene many of its desirable properties.

A ball-and-stick model of syndiotactic polypropylene.

More precisely engineered Kaminsky catalysts have been made, which offer a much greater level of control. Based on metallocene molecules, these catalysts use organic groups to control the monomers being added, so that a proper choice of catalyst can produce isotactic, syndiotactic, or atactic polypropylene, or even a combination of these. Aside from this qualitative control, they allow better quantitative control, with a much greater ratio of the desired tacticity than previous Ziegler-Natta techniques. They also produce narrower molecular weight distributions than traditional Ziegler-Natta catalysts, which can further improve properties.

To produce a rubbery polypropylene, a catalyst can be made which yields isotactic polypropylene, but with the organic groups that influence tacticity held in place by a relatively weak bond. After the catalyst has produced a short length of polymer which is capable of crystallization, light of the proper frequency is used to break this weak bond, and remove the selectivity of the catalyst so that the remaining length of the chain is atactic. The result is a mostly amorphous material with small crystals embedded in it. Since each chain has one end in a crystal but most of its length in the soft, amorphous bulk, the crystalline regions serve the same purpose as vulcanization.

Mechanism of metallocene catalysts

The reaction of many metallocene catalysts requires a co catalyst for activation. One of the most common co catalysts for this purpose is Methylaluminoxane (MAO). Other co catalysts include, Al(C2H5)3.There are numerous metallocene catalysts that can be used for propylene polymerization. (Some metallocene catalysts are used for industrial process, while others are not, due to their high cost.) One of the simplest is Cp2MCl2 (M = Zr, Hf). Different catalyst can lead to polymers with different molecular weights and properties. Active research is still being conducted on metallocene catalyst.

In the mechanism the metallocene catalyst first reacts with the co catalyst. If MAO is the co catalyst, the first step is to replace one of the Cl atoms on the catalyst with a methyl group from the MAO. The methyl group on is replaced by the Cl from the catalyst. The MAO then removes another Cl from the catalyst. This makes the catalyst positively charged and susceptible to attack from propylene.

Once the catalyst is activated, the double bond on the propene coordinates with the metal of the catalyst. The methyl group on the catalyst then migrates to the propene, and the double bond is broken. This starts the polymerization. Once the methyl migrates the positively charged catalyst is reformed and another propene can coordinate to the metal. The second propene coordinates and the carbon chain that was formed migrates to the propene. The process of coordination and migration continues and a polymer chain is grown off of the metallocene catalyst.

Manufacturing

Melt processing of polypropylene can be achieved via extrusion and molding. Common extrusion methods include production of melt blown and spun bond fibers to form long rolls for future conversion into a wide range of useful products such as face masks, filters, nappies (diapers) and wipes.

The most common shaping technique is injection molding, which is used for parts such as cups, cutlery, vials, caps, containers, houseware and automotive parts such as batteries. The related techniques of blow molding and injection-stretch blow molding are also used, which involve both extrusion and molding.

The large number of end use applications for PP are often possible because of the ability to tailor grades with specific molecular properties and additives during its manufacture. For example, antistatic additives can be added to help PP surfaces resist dust and dirt. Many physical finishing techniques can also be used on PP, such as machining. Surface treatments can be applied to PP parts in order to promote adhesion of printing ink and paints.

Applications

Polypropylene lid of a Tic Tacs box, with a living hinge and the resin identification code under its flap

Since polypropylene is resistant to fatigue, most plastic living hinges, such as those on flip-top bottles, are made from this material. However, it is important to ensure that chain molecules are oriented across the hinge to maximize strength.

Very thin sheets of polypropylene are used as a dielectric within certain high performance pulse and low loss RF capacitors.

High-purity piping systems are built using polypropylene. Stronger, more rigid piping systems, intended for use in potable plumbing, hydronic heating and cooling, and reclaimed water applications, are also manufactured using polypropylene. This material is often chosen for its resistance to corrosion and chemical leaching, its resilience against most forms of physical damage, including impact and freezing, and its ability to be joined by heat fusion rather than gluing.[citation needed]

A polypropylene chair

Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an autoclave. Its heat resistance also enables it to be used as the manufacturing material of consumer-grade kettles. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminum foil (both heat-resistant materials). After the product has cooled, the tubs are often given lids made of a less heat-resistant material, such as LDPE or polystyrene. Such containers provide a good hands-on example of the difference in modulus, since the rubbery (softer, more flexible) feeling of LDPE with respect to PP of the same thickness is readily apparent. Rugged, translucent, reusable plastic containers made in a wide variety of shapes and sizes for consumers from various companies such as Rubbermaid and Sterilite are commonly made of polypropylene, although the lids are often made of somewhat more flexible LDPE so they can snap on to the container to close it. Polypropylene can also be made into disposable bottles to contain liquid, powdered or similar consumer products, although HDPE and polyethylene terephthalate are commonly also used to make bottles. Plastic pails, car batteries, wastebaskets, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature.

Polypropylene is a major polymer used in nonwovens, with over 50% used[citation needed] for diapers or sanitary products where it is treated to absorb water (hydrophilic) rather than naturally repelling water (hydrophobic). Other interesting non woven uses include filters for air, gas and liquids where the fibers can be formed into sheets or webs that can be pleated to form cartridges or layers that filter in various efficiencies in the 0.5 to 30 micron range. Such applications could be seen in the house as water filters or air conditioning type filters. The high surface area and naturally hydrophobic polypropylene nonwovens are ideal absorbers of oil spills with the familiar floating barriers near oil spills on rivers.

A common application for polypropylene is as biaxially oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear bags. When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products.

Polypropylene, highly colorfast, is widely used in manufacturing rugs and mats to be used at home.

In New Zealand, in the US military, and elsewhere, polypropylene, or ‘polypro’ (New Zealand ‘polyprops’), has been used for the fabrication of cold-weather base layers, such as long-sleeve shirts or long underwear (More recently, polyester replace polypropylene in these applications in the U.S. military, such as in the ECWCS ). Polypropylene is also used in warm-weather gear such as some Under Armour clothing, which can easily transport sweat away from the skin. Although polypropylene clothes are not easily flammable, they can melt, which may result in severe burns if the service member is involved in an explosion or fire of any kind.

Polypropylene is widely used in ropes, distinctive because they are light enough to float in water. For equal mass, polypropylene rope is the same strength as plow steel wire rope. Polypropylene rope is stronger than nylon, but not as strong as polyester. Likewise, it is more UV resistant than nylon, but not as much as polyester. It usually costs less than polyester and is not as elastic as nylon.

Polypropylene is also used as an alternative to polyvinyl chloride (PVC) as insulation for electrical cables for LSZH cable in low-ventilation environments, primarily tunnels. This is because it emits less smoke and no toxic halogens, which may lead to production of acid in high temperature conditions.

Polypropylene is also used in particular roofing membranes as the waterproofing top layer of single ply systems as opposed to modified bit systems.

Its most common medical use is in the synthetic, nonabsorbable suture Prolene, manufactured by Ethicon Inc.

Polypropylene is most commonly used for plastic moldings where it is injected into a mold while molten, forming complex shapes at relatively low cost and high volume, examples include bottle tops, bottles and fittings.

Recently it has been produced in sheet form and this has been widely used for the production of stationery folders, packaging and storage boxes. The wide color range, durability and resistance to dirt make it ideal as a protective cover for papers and other materials. It is used in Rubik’s cube stickers because of these characteristics.

The availability of sheet polypropylene has provided an opportunity for the use of the material by designers. The light-weight, durable and colorful plastic makes an ideal medium for the creation of light shades and a number of designs have been developed using interlocking sections to create elaborate designs.

Polypropylene sheets are a popular choice for trading card collectors; these come with pockets (nine for standard size cards) for the cards to be inserted and are used to protect their condition and are meant to be stored in a binder.

Polypropylene has been used in hernia and pelvic organ prolapse repair operations to protect the body from new hernias in the same location. A small patch of the material is placed over the spot of the hernia, below the skin, and is painless and is rarely, if ever, rejected by the body. However the FDA has issued several warnings on the use of polypropylene mesh medical kits when used for certain applications in pelvic organ prolapse, specifically when introduced in close proximity to the vaginal wall due to a continued increase in number of mesh erosions being reported by patients over the past few years.

The material has recently been introduced into the fashion industry through the work of designers such as Anoush Waddington who have developed specialized techniques to create jewelry and wearable items from polypropylene.

Polypropylene is used in the manufacture of loudspeaker drive units. Its use was pioneered by engineers at the BBC and the patent rights subsequently purchased by Mission Electronics for use in their Mission Freedom Loudspeaker and Mission 737 Renaissance loudspeaker.

In 2008, Researchers in Canada asserted that quaternary ammonium biocides and oleamide were leaking out of certain polypropylene labware, affecting experimental results. Since polypropylene is used in a wide number of food containers such as those for yogurt, Health Canada media spokesman Paul Duchesne, said the department will be reviewing the findings to determine whether steps are needed to protect consumers.

Expanded Polypropylene (EPP) is a foam form of polypropylene. EPP has very good impact characteristics due to its low stiffness, this allows EPP to resume its shape after impacts. EPP is extensively used in model aircraft and other radio controlled vehicles by hobbyists. This is mainly due to its ability to absorb impacts, making this an ideal material for RC aircraft for beginners and amateurs.

Recycling

Polypropylene is commonly recycled, and has the number “5″ as its resin identification code: .

References

^ Peter J. T. Morris (2005). Polymer Pioneers: A Popular History of the Science and Technology of Large Molecules. Chemical Heritage Foundation. p. 76. ISBN 0941901033. http://books.google.com/books?id=GjtJfmxvSWgC&pg=PA76. 

^ This week 50 years ago in New Scientist, 28 April, 2007, p. 15

^ R. Kleinschmidt et al. Journal of Molecular Catalysis A: Chemical, 157(2000)8390

^ Kyung-Jun Chu. Eur. Polym. J. Vol. 34, No. 3/4, pp. 577-580, 1998

^ Session 6

^ Song (2004). “Zirconocene-catalysed propene polymerisation: kinetics, mechanism, and the role of the anion”. Macromol. Symp. 213: 173185. doi:10.1002/masy.200450917. 

^ P. Mercandelli; Sironi, Angelo; Resconi, Luigi; Camurati, Isabella (2007). “Comparing propene polymerization with 1-butene polymerization catalyzed by MAO-activated C2- and C1-symmetric zirconocenes: An experimental and computational study on the influence of olefin size on stereoselectivity”. Journal of Organometallic Chemistry 692: 47844791. doi:10.1016/j.jorganchem.2007.06.021. 

^ ASTM Standard F2389, 2007, “Standard Specification for Pressure-rated Polypropylene (PP) Piping Systems”, ASTM International, West Conshohocken, PA, 2007, DOI: 10.1520/F2389-07E01, www.astm.org.

^ Rug fibers

^ ECWCS Gen. III

^ USAF Flying Magazine. Safety. Nov. 2002.

^ Rope Materials

^ Plastic additives leach into medical experiments, research shows, Physorg.com, 10 November 2008

^ Scientific tests skewed by leaching plastics, November 6, 2008.

^ Plastics recycling information sheet, Waste Online

External links

Wikimedia Commons has media related to: Polypropylene

Chain structure of Polypropylene

Technical Properties & Applications

Polypropylene is traded on the London Metal Exchange

v  d  e

Plastics

Polyacrylic acid (PAA)   Cross-linked polyethylene (PEX or XLPE)   Polyethylene (PE)  Polyethylene terephthalate (PET or PETE)  

Polyphenyl ether (PPE)   Polyvinyl chloride (PVC)  Polyvinylidene chloride (PVDC)  Polylactic acid (PLA)  Polypropylene (PP)  Polybutylene (PB)  Polybutylene terephthalate (PBT)  Polyamide (PA)  Polyimide (PI)  Polycarbonate (PC)  Polytetrafluoroethylene (PTFE)  Polystyrene (PS)  Polyurethane (PU)  Polyester (PEs)  Acrylonitrile butadiene styrene (ABS)  Poly(methyl methacrylate) (PMMA)  Polyoxymethylene (POM)  Polysulfone (PES)  Styrene-acrylonitrile (SAN)  Ethylene vinyl acetate (EVA)  Styrene maleic anhydride (SMA)

Categories: Dielectrics | Packaging materials | Plastics | Polymers | Polyolefins | Speciality films | ThermoplasticsHidden categories: Chemboxes which contain changes to watched fields | All articles with unsourced statements | Articles with unsourced statements from June 2009 | Articles with unsourced statements from March 2009
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